Secondary batteries such as a lead-acid battery, a nickel-hydrogen battery, or a lithium-ion battery are mounted in a vehicle that travels using electricity as motive power. Electric power required when a hybrid automobile or an electric automobile travels is supplied from these secondary batteries.
Secondary batteries experience capacity deterioration due to aging (that is, the storage capacity decreases). SOH (State of Health, deterioration state) of a secondary battery is an index of the capacity deterioration. PTL 1 discloses the following method of calculating battery capacity after deterioration. Two time points at which open-circuit voltages are obtained are selected, and SOCs (State of Charge, charge rate, charge state) and a SOC difference are calculated from the open-circuit voltages. The battery capacity after deterioration is obtained by dividing a time integrated value of a charge and discharge current in a period defined by the two time points by the SOC difference. The two time points are a predetermined time point after an ignition-ON point and a predetermined time point after an ignition-OFF point. A capacity maintenance rate (SOHQ) which is one of SOH indices can be calculated by dividing a secondary battery storage capacity obtained by the invention disclosed in PTL 1 by an initial secondary battery storage capacity.